The present invention relates to a hip joint prosthesis and more particularly to the femoral component of such a prosthesis.
Many methods and devices have been developed to improve the fixation of hip joint prostheses including the femoral component thereof in the body so that the device implanted therein becomes as permanent as possible. Many orthopedic implants use a cement to anchor the stem portion of a femoral component in the femur. For example, United Kingdom Patent Specification No. 1,409,054 in the names of Robin S. M. Ling (one of the co-inventors of the present application) and Alan J. C. Lee (incorporated herein by reference) discloses a hip joint prosthesis having a double-tapered stem which, among other advantages, enhances extrusion of cement caused by penetration of the stem during fixation. U.S. Pat. No. 3,793,650, (incorporated herein by reference) one of the co-inventors of which is also Robin S. M. Ling, discloses an intramedullary stem for a prosthetic bone joint device having a base with spring members intended to centralize the position of the stem in the canal or bore of the bone in order to insure a relatively uniform or, at least minimum thickness of cement between the wall of the bone and the stem. It is desirable that there be at least two millimeters (2 mm) of cement between the stem and the bone. By providing a means for insuring that there will be at least a certain minimum thickness of cement between the stem of the prosthesis and the interior wall of the canal formed in the femur bone for receiving such stem, the likelihood of the stem protruding through the cement and contacting the interior of the femur bone itself is minimized. Thus, in those types of implants using cement, it is important to insure that the stem is completely encapsulated by the cement and does not protrude through to contact the bone.
One type of bone cement utilized to retain the stem of a femoral hip joint prosthesis in the canal of a bone comprises a mixture of polymethylmethacrylate (hereinafter PMMA) polymer and methyl methacrylate monomer and optionally including a styrene co-polymer of PMMA. This and other types of cement utilized for such purpose may be packaged in two separate components which are mixed into a paste which is placed in the canal of the femur immediately prior to insertion of the stem of the prosthesis. Such paste then sets to a relatively rigid material providing excellent adherence to the interior wall of the bone.
Heretofore, it has been the belief that it is desirable to have good adhesion between the stem and the cement. Many prior art devices were specifically directed to providing a design for the prosthesis intended to maximize adhesion between it and the cement. For example, the CML Cemented Medullary Locking Hip System manufactured by DePuy Division of Boehringer Mannheim Corporation, Warsaw, Indiana, is a hip system in which the upper portion of the stem is provided with a roughened textured surface intended to enhance the bond of the cement to the prosthesis at the prosthetic interface. It also utilizes a "Macro-Textured" waffle design which is intended to increase the surface area and the mechanical interlock between the cement and the prothesis in the area of such waffle design.
Osteonics Corp., Allendale, N.J., manufactures the OMNIFLEX Femoral System of a titanium alloy having a normalized surface to promote good adhesion of the cement thereto.
U.S. Pat. No. 4,281,420 (incorporated herein by reference) is directed to maximizing the strength and durability of the prosthesis/cement adherence.
Other types of devices which disclose the use of cement within a bore or canal of the femur are described in U.S. Pat. Nos. 3,829,904; 3,874,003; 4,012,796; and 4,080,666, the disclosures of which are hereby incorporated by reference. Copies of such patents are enclosed.
Still other types of prostheses are intended for use without cement. Many of these are designed to provide a porous or roughened surface in order that the bone may grow into the porous surface of the prosthesis. For example, U.S. Pat. No. 3,808,606 discloses a prosthesis possessing porous surfaces for fixation by tissue ingrowth. U.S. Pat. No. 4,164,794 discloses prosthetic devices having outer foamed or sintered porous coatings of selected bioengineered thermoplastics which enables the device to become firmly anchored to the bone by tissue ingrowth into the coated material.
In both the cemented and non-cemented types of devices used heretofore, problems have arisen, particularly after a number of years of implantation. With respect to the cemented type devices, part of the problem arises from the fact that the cement utilized to retain the stem of the device in the canal of the femur bone is subject to a phenomenon known as creep. Thus, while the bone cement appears to be rigid when set, it is subject to minute amounts of movement over time. The amount of creep encountered with such cement following implantation is exaggerated by virtue of the fact that the body temperature controls the temperature of the implanted cement and prosthesis. Thus, PMMA and other types of bone cement at body temperature are subject to a greater degree of creep than bone cement maintained at room temperature of, say, 72.degree. F. This may be readily observed by mounting a bar of PMMA so that its ends are supported and applying a fixed load at the center of the bar. Tests have shown that a bar so supported and subjected to a load of 5 pounds for eight hours at 98.6.degree. F. will deflect to an extent 3.5 times greater than an identical bar supported and loaded in an identical manner for eight hours at 72.degree. F.
Over a period of time, the phenomenon of creep may result in disruption of the micro-interlocking of the cement-bone interface, especially if the cement mantle is firmly bonded to the femoral prosthesis. As is well known in the field of hip replacements, it is important that there be a good bond between the cement and the bone and that there be no disruption in the micro-interlocking of the cement-bone interface.
Subsidence of the femoral component occurs in various degrees with prostheses of different designs regardless of the presence or absence of collars. Any firmly bonded or fixed prosthesis to the cement will disrupt the cement bone interface which will inevitably lead to clinical loosening and subsequent failure necessitating revision.